Hi all, new here. This is going to be slightly long as it's going to be a catch-all thread for all my questions. I'm not new to modifying cars and will be doing all work myself. A lot of these questions are more basic than I'd like, but a lot of threads reference resources that are no longer available or have pictures that no longer load. Going forward, assume I'm budget conscious, but willing to spend where required.

Few months back I purchased a 1989 Corvette ZF-6 with the intent of it being a project car and modifying it for more power (more on that later). Car is essentially stock and has the Z51 package. It's running 22lb delphi injectors and has a (crappy) cat or axleback. It also looks like the secondary cat was replaced at some point (unsure if high flow or not). Finally, I'm not looking to turn this into a project that spends months under the knife before driving. I don't expect to get everything done in a weekend, but it'd be nice to do things in stages or without much downtime. I have no interest in drag racing, think canyon carving/autox type stuff being the goal.

Power. I'm looking to increase the top end. I want to be clear, I am not slightly interested in a torque monster build. I'm happy with the torque, but I'd even be willing to lose some for the desired powerband. Early goal is as close to 300whp as possible. So, how? I've identified several options for starting at this and some corresponding questions. So I have 3 options on my mind.

1. N/A with L98: TPIs Mini Ram, cam (which one for top end), Heads (which for top end), and eventually headers (open to suggestion)
   1. I'd look into the LT1 intake, but I have not been able to find a guide on how to modify it, and the website everyone refers to seems shut down as of a month ago. If someone could point me to clear instructions, I'd appreciate it.
   2. Fuel. To what power can I expect 22lb injectors to take me before needing to upgrade? What about the fuel pump?
   3. Tune. I understand I'll either need a new chip to replace, or a flashable one. I've seen many sources for these. Right now I'm leaning TPIs. Two questions: At what point is it necessary, and who is recommended?
   4. With my plan above, what are reasonable expectations of power, and what is missing? Finally is that path conducive to adding forced injection in the future if I desire
2. Supercharging. This would easily get me to my goal, and exceed it, at large expense. Not opposed, but I did want to do more than bolt on a procharger. May want to do this after everything else in #1 if it doesn't get me there.
3. Engine swap. Thinking aluminum truck LS with appropriate mods to gain top end. Struggling with a consistent resource since literally every post has "wHy NoT jUsT bUy A c5?" Probably the least cost effective option. However, it would bring a more modern engine into the mix. My sticking point here id really that I can't find a consistent source for things to consider that are specific to the C4. I've read a lot of forum posts and seen a bunch of videos for LS swaps on other cars, so I know what's generally involved and I'm capable. It's just not something I want to get into and "surprise $5k worth of extra things to do that you didn't plan for because of incomplete information." This is lowest on the list because I also know it'd take a long time to do, but still compelling.

I'm strongly leaning #1 so I'm hoping to receive a lot of input on that. Oh, and finally, everything that must be done must be doable with hand tools and jack stands. No machine shops, no welding. I'm not particularly intrested in N2O or meth injection either...but could be convinced once other things have been done.

Suspension and Handling
It isn't bad at all, but most bushings are due and the shocks are definitely due for replacement. Current plan is just factory replacement since the Z51 kit is pretty good, but I'm curious for suggestion if there are legitimate upgrades to the worn components that I should consider. I haven't seen much talk about this (read quite a few threads). Oh, I will not consider any solution that lowers the car, it'd look good, but it's already a pain as it is stock.

Brakes (Possible troubleshooting)
I have a steep driveway, and when rolling down in reverse, ABS sometimes activates in the rear (with ABS light in console) causing loss of braking power in rear only. Any suggestions for cause? Car seems to stop fine, but I haven't tested emergency stopping or threshold braking yet. Could be normal, could be a problem, just looking for confirmation since I've never had this happen in any other car.

If you've made it this far, thanks! I look forward to the feedback and being part of the community. I also want to restate, in earnest, that I did buy this car to be a project, so work is not a problem and I look forward to it.

 

If the goal is only 300rw, option 1 will have no problem getting there. I would beef the bottom end up for option 2, and I would not personally consider option 3.

If you intend to do nearly all yourself at home, I'll assume you will stay as a 350 SBC, in which case I'd probably go with AFR 180-185 heads. 195s if you want the flexibility to go with a 383 or 396 later. I personally run 195s in a 398 now. Your stock heads can be ported out to support about 350bhp, so pretty close to your 300rw goal as they are. Its generally accepted here over the years that the '113 head is pretty good, but not what you'd use for an "ultimate" build.

Swapping on the MR now on the stock engine basically makes you into an LT1 engine, so you'll be around 300bhp with it. It would not be absolutely necessary to tune then, but you would have dead spots with the factory A/F settings, so if you stopped there, tune. If you are going to dig in and do the cam 6 months later, deal with some dead spots and tune the chip later.

Camshaft change would end up necessitating the chip to be reprogrammed, preferably on a dyno by a tuner. Don't do off the shelf stuff unless its a known, common combo. Get it right on a dyno the first time. The tuner would really be the one to talk to about the fuel injector size, but if 300rw is the goal, the stock size could support it, but I'd still upgrade to the newer Bosch III in the size he recommends. 24 should be plenty, but talk to the tuner who will do the programming for you. Same deal for the stock pump, it could get you to 300rw, but I wasn't comfortable with that and went with a Walbro 255 pump from Racetronix, and if you go that route, you should upgrade the in-tank wiring harness along with their piggyback pump harness system.

I'd recommend TPiS headers, 1 3/4", coated. Hooker 2149/2151 are also good headers. There are other brands but I always recommend those two for fit/finish. You do not need to upgrade the factory starter in 89.

The cam you pick will get you to your goal of 300rw, depending on what kind of drivability you want, you can go bigger or slightly smaller, but I'd probably go with something in the 220/230 @050 range with along with 1.6 RR. That, the MR, and headers, and finally the chip reburn, and you'll get over 300 at the wheels.

Given that its an Miniram'd car, a rear gear upgrade wouldn't upset you with the ZF6 you have. 4.10 would be the choice.

Suspension, your shocks are probably out or on their way from age. Bilsteins are what I'd recommend, if you have money burning a hole in your pocket, RideTechs. You could also upgrade some parts with Banski pieces. But, shocks and new rubber bushings are what I'd start with for a car I drove around the mountains a lot, and you'd probably be happy.

The rear braking issue sounds like a problem and not due to your driveway, but I've never had the rear ABS engage on me going backwards on a steep drive before.

 

As far as LS swap, 89 is easy, you may lose abs but im not 100% sure on that. Otherwise you only lose the fuel economy readouts on the digital dash. That bekng said a clean install is going to be time consuming to a point unless that doesn't matter. Then there is the bellhousing on the zf that has to be adapted too iirc.

The other plan... top end. Intake cam and heads would probably put you closer to 400rwhp depending on cam selection. Something in the 224/230 range will give you an upper end in the 6400 range with good power 2000 and up. You'd probably be 375 with something like that. I'd personally go with a 220/224 like said above. That would put you around 1700-5800 for a usable band.

Heads: nothing crazy. Afr if you want to spend the coin. Otherwise profiler/jegs or brodix or dart. Any of the mid teir aluminum heads in a 180-195 cc range will be fine.

Intake: already taken care of i see

injectors: if you end up in the 300 something range you'll probably want a 26-30lb/hr. This is something you ask your tuner about.

 

Wow, thanks for the quick and comprehensive reply vader. In my research I've seen a lot of your posts. This is some great feedback. 300rwhp is definitely a starting point, based on cars I've had in the past, I may end up slightly higher, but time will tell. Had a tuned LT1 in an 87 944 and a '16 Mustang GT w/PP and was totally satisfied with both on the acceleration front. Still miss the 944 and spend way too much time perusing listings to see if it has popped back up.


Edit: next paragraph is pointless; You already addressed it in your post and my brain omitted it despite several readthroughs
I really only have one question to your feedback, since you touched on literally everything else;
Do you have a cam suggestion? I see ZZ# often, but this is where my ignorance to the finer details of GM products shines. I've also never done anything with camshafts before, though I totally get the core concept. Drivability is not a huge concern, though I'd stop short of the car being a handfull. It'll be a street car, but nowhere close to dailied.

Again, thanks. It's really helpful for me to know I wasn't far off on my thoughts for #1

 

Thanks 84 4+3! That's fantastic feedback too! I was not aware that such a power range was even possible N/A...Really glad to read I was not far off on my thoughts even though I had open questions on how to execute.

My question for you would be, (aside from the time) what makes an 89 easy for LS swap? Based on what I've read here, unless something drastic happens I'll be going with the top end build...but I'd still like to know.

 

mainly, you don't have all the electronic goodies to deal with like the later lt1 cars do. Basic hvac, basic dash, etc.

 

All, thanks again for the advice. Been doing some additional poking around on the web, though not as much as I'd like since work has been crazy lately. Before I start ordering parts a few quick questions. Apologies for the questions that have been answered prior.

1. The miniram. I see options for a 1204, 1205, and fast burn. I've read 1204 is the factory gasket size for the L98, but unless I've misunderstood other posts is there a reason I'd want to go 1205 given the info in this thread?
2. Heads. Been looking at 84's recommendations as a budget option to save cash and the guides on every vendor's site have left me scratching my head (still a novice but I learn quick- I'm better with modern turbo engines). I'm going with 185s. I know I need 23 degree heads. I've read that I should go with angled plugs. Questions are based off Profiler's website (again, still learning SBC)Honestly, I think I'm overthinking things and making it more complicated than it is...but it's my nature.
To the braking issue I referenced, they are fully operable. Only tested a little, but the car doesn't pull in any direction under braking, I can threshhold brake to what I consider satisfactory performance, and when I mash the stoppy pedal with reckless abandone it activates ABS with proper functionality. Going to leave it as-is for now until I have more data to work with.

 

Budget?

 

At least for this year...As far under 3k as possible. The leftovers go to getting a seat that stops me hitting my head every time I hit a large bump (I've already gutted the headliner and removed the straps beneath the seats). Based on this thread it seams reasonable to hit initial my goal of near 300whp for around 2k.

 

Tight seatbelts will keep you from hitting roof.88 91 Z51 will beat you to death.it's great on smooth roads absolutely sucks on "road patch". 94-96 base seat on manual adjuster will give more headroom. Horsepower don't forget to factor in tuning, unless you are moving to carb. Carburetor usually requires hood scoop.

 

Unfortunately this isn't quite something a seatbelt could fix. A harness could, but I'm not strapping myself down like that on the street for both safety and comfort reasons.

As for tuning it was discussed above and I fully intend on going that route. Still just struggling with my questions to the community on some of the finer details for hardware. I'm close to reaching out to the vendors for their input, but I'd prefer input from (theoretically) neutral parties who've done this with their cars first.

 

For as close to 300rwhp as possible, honestly just the Miniram and a hot cam with the stock heads, and maybe long-tube headers would get you there. A stock L98 is good for 220-230rwhp. The intake alone will probably get you 25-30 more by extending your useful powerband another 1000rpm. The cam is probably good for another over the stock L98 cam. So you're closing in on 280-290rwhp just with that. Headers with a decent exhaust is probably worth another 10-15, and there you are. If I were in your shoes, I might leave the stock heads and save for a year to get AFR 195 Comps. That would give you a great base for however much more power you eventually want to have. You can probably sell the hot cam kit for half what you paid for it if it remains in good condition, and then get something like a Comp Cams 268 XFI. That with the Miniram would put you at more like 350-360rwhp, I think - fast enough to be pretty fun. I'd skip the supercharger altogether, but that's just me. I went the other way: no replacement for displacement, with a 396 stroker.

As for suspension, you can't get new stock rubber bushings. The best approximation of them is the SuperPro purple urethane bushings. They are softer than standard hard poly, which you need in the rear links (otherwise you get bind). The other option in back is rod-end links like Banski sells, but for a pure street car (no autocross or track stuff) the SuperPros are probably great. I would recommend spending good money on shocks. No KYB or Monroe crap. I know the Bilsteins are good quality, but I've never been a big fan of their valving choices, and they aren't adjustable. But they are the bare minimum I'd recommend. Honestly, I would recommend the Ridetech HQ single-adjustable shocks all day every day. They are very high quality monotube dampers with Fox internals, and the adjusters are easy to get to. On the softer settings they have very good ride quality by all accounts, and if you ever want to autocross or track the car, or even divebomb your favorite curvy road, you can stiffen them up for great motion control. You will get what you pay for in this aspect (~$700/set). Check the ball joints and tie rod ends as well as the rear toe link joints - none of those are expensive to replace if you do the work yourself.

On brakes, it kind of sounds like you may have a jacked up wheel speed sensor in one or both rear hubs. That's a strange symptom. I doubt it's actually your brakes. However, if yours are old or getting thin, I'd replace all pads with something decent-but-inexpensive, like Powerstop z26 pads. Either have your rotors turned or replace them with plain ones from Centric or some other basic brand - no need for slots and holes and other doodads. Flush your brake fluid and use good replacement fluid: Castrol DOT 4 at a minimum, or Motul 600 is better yet. Inspect all your hoses and make sure they are good, and grease your caliper pins and pad contact points. See how things work after that. Oh, and check your wheel bearings (hubs) front and rear - C4s are notorious for wearing those out, and that could play hell with your handling and brakes.

 

It is not easy if you want to keep air conditioning.

 

By any chance is this your first Corvette? I always suggest to folks with a new to them Corvette to "Learn" the car. It takes time to do this and driving to understand the weak spots in the Corvette. Lots of us have had other performance cars, Corvettes are a different ball of wax. There are several tools that are important to have to work on the C4's. Be sure to get the Factory Service Manual, a good Volt Ohm meter and an external Fuel pressure gauge to start with.

A lot of people buy a C4 Corvette and suddenly feel that they want/need more horsepower. The C4 was designed for normal driving around towns and highways. It is a great overall Corvette. I have a 1988 C4 and it has a Chambered Exhaust system on it and with the high flow catalytic gained almost 14 hp. The C4 is a great Corvette all by itself, it may not be the most powerful Corvette ever offered but it is a joy to drive a nice tight well-maintained C4. Your 31 year old Corvette is not going to outrun the new HellCats and unless you totally de-engineer it and go back to a solid rear axle the C4's are not going to like the power.

Get a small pad of paper and keep it in the Corvette. While driving the Corvette jot down your thoughts or questions. Make a list of things that you think need attention and start fixing up the Corvette's suspension and get it nice and tight like it was when built. I spent a lot of time and money on my C4 getting it back into shape. I spent a lot of money getting the rear suspension back in shape and it took several months of hard work. My C4 runs great and is a pleasure to drive on the highways, not to mention my L98 with a 700R4 gets over 30 mpg on the interstates.

I still have my first Corvette, a 1968 C3 with a 427. There is enough power in that engine to explode the u-joints in my half shafts. Having power is great but what good is it if you can't get it onto the pavement? My C3 will burn the rear tires for 1/8th mile on a quarter mile track and good tires. I wish they had made a big block version for the C4, that would have been a frame twisting experience.

Be Well, Stay Well!

 

Thanks a bunch for this.
For the brakes, that's some good feedback. Current brakes are in good condition right now, but the suggestions are appreciated. Will double-check wheel bearings. Hadn't considered a wheel speed sensor, so I'll look into what's going on in that regard. Have looked up nothing regarding those, but depending on cost and difficulty I'll either just replace them or ignore until I have narrowed it down.

Suspension, also great input. I've seen poly bushing kits around, but I wasn't aware of that detail you threw out regarding binding. Honestly haven't done too much consideration of what I want to do with the bushing situation yet. For shocks, in all likelihood I'll stick with Bilstein, but it's good to know that there's some readily available quality out there that's adjustable.

On the power mods, thanks here as well. Hadn't considered the hotcam, mostly because I wasn't aware of it, but I'll research further. I'm assuming that's the LT4 hotcam I 've just now seen some threads on. I've been debating between the incremental approach and "just do it all while I have it apart" approach. After reading this I believe I'm going to go incremental with a cam and miniram and enjoy my near 300whp for a bit. In a car this weight with the powerband it'd have it'll have the desired performance for the time being and allow me to focus on the rest (suspension, brakes, seats).

 

First vette. I picked the C4 because I like them, but also knew it before buying that it did not quite offer the performance I want. Before buying I was decided that it needed more power. For me it was simply, "I want a project, I love how the C4 looks, handling is decent, power could use a bump." If I wanted the easy path for performance I'd just have bought a much newer vette or something else. Not looking to win any races either, not really about that. I'm about cornering fun as much as I am about power (mostly to increase the giggles on corner exit/between corners).

 

Yes the hotcam is the LT4 hotcam referred to in nearly all Gen 2 LT1 build threads at some point. You can use it, its basically in the 220/230 range I recommended.

 

Update to this thread. Have my miniram on order. Have decided to grab the TPIS headers and their chip (as a temporary measure) and ride like that until next year. Unfortunately, it seems TPIs is shut down due to the virus, so the miniram hasn’t shipped despite ordering a few weeks back, so everything is on hold til then. I’d have ordered the headers and all that by now if it did. Once the tpis parts are installed, I’ll be working other maintenance items before heads/cam.

 

The ABS does a self check after starting the car when it reaches 3 mph. Never paid attention on the early cars if it activates the light or not when it does this. But rolling downhill and having it go through the self check will probably give you the symptoms you are experiencing.

 

Another update, Corvette down (temporarily).

Took it out for a fun backroads drive on Friday, and things went south. Ambient temps were approximately 90F with high humidity. Had sudden loss of power, and noticed that oil was hot, approaching 255F, and coolant was around 225. Immediately started towards home. Noticed no smoke. Made it into the garage without temperature climbing further from prior numbers, shut it down. The heat coming off the engine was intense enough that I decided to look into it the next day.

Tried to drive on Saturday, just to see if it was temp related. No improvement, got home and shut it down before it warmed up. Also thought I heard a faint “induction” type sound under throttle, but can’t be sure.

Noticed oil on the ground from after my drive on Friday. Not a huge amount, but enough to be concerned. Both passenger and a drivers side are wet at rear of block under the headers. Not the greatest pics, but they served their purpose to show me the ballpark of where the oil was coming from.



Have not had chance to drain some oil to check for coolant, but it passed the dipstick test and looking in the oil cap showed nothing abnormal- but that doesn’t mean much. Have some ideas, but advice would be appreciated. Have done no additional disassembly or troubleshooting beyond what is described here. Really need to clean and reorganize my garage so I actually have space to work with...